Many variants of plastic films are known. Hot melt adhesive films are a particular embodiment. Hot melt adhesive films, their properties and advantages, together with processes for the production thereof from conventional granulated and powdered hot melt pressure sensitive adhesives are described, for example, in U.S. Pat. No. 4,379,117 (Grace) or DE 2 114 065 (Plate Bonn). DE 3 911 613 (Audi), for example, describes the use of hot melt adhesive films. A general classification of adhesives may be found in Habenicht, Kleben, Springer Verlag, Berlin 1986. Hot melt adhesive films are known in embodiments as films having adhesive properties on both sides as well as laminating films having adhesive properties on only one side.
According to the prior art, films consisting of at least one hot-bonding, low-softening layer of a polymer resin and at least one higher-softening layer of a second polymer resin are known as laminating films. Film structures consisting of at least one low-softening and one higher-softening layer are known from the packaging sector, in particular from food packaging. The films used in this sector are usually produced from polyolefins, which have only low hot-bonding properties. In contrast with the use of laminating films, in which a bond is formed between a film with adhesive properties on one side with inter alia a non-adhesive substrate, in heat sealable packaging two heat sealing layers are fused together.
Laminating films thus differ from pressure sensitive adhesive films produced from a support which is coated with a pure pressure sensitive adhesive, as are for example described in WO 92/22619 (Minnesota Mining & Manufacturing Company). It is not only at elevated temperatures that these films offer inadequate adhesive power and they may moreover too readily be detached from the substrate.
Known applications for laminating films is the provision of a skin on porous articles, such as for example foams, woven goods and non-woven materials. The purpose of providing this skin is inter alia to produce a smooth surface and thus provide protection against the penetration or passage of fluids, generally to provide protection against water. Foam systems may thus be protected inter alia against rotting by the provision of a skin. The articles provided with a skin in this manner may moreover readily be printed or painted, which is not straightforwardly possible with porous surfaces.
In industrial applications, the properties of the widely used and low cost polyolefins are often insufficient, such that laminating films made from engineering thermoplastics such as polyamides, polyesters or also combinations thereof must be used. This is particularly the case when substrates with polar surfaces are to be covered or when not only the adhesive, but also the cohesive properties of the adhesive are of decisive importance. If the laminating films are additionally intended to have elastic properties, polymer resins from the group of thermoplastic polyurethanes (TPU) are readily used for the layer which softens in the higher temperature range. This group includes both low-softening grades, so-called hot melts, and higher-softening standard grades.
TPU are readily used as the higher-softening layer in known elastic laminating films made from thermoplastic materials as they offer a wide range of relevant properties, such as elevated abrasion resistance and elevated tear strength combined with elevated elongation at break and good resilience. Commercially available TPU are in principle suitable as the higher-softening layer of laminating films, in particular those TPU which are already used for single-layer films or for the raw material formulations on which they are based.
Single-layer films made from higher-softening TPU, processes for the production thereof and the use thereof are known according to the prior art, for example from EP 0 308 683 (BF Goodrich), EP 0 526 858 (Elf Atochem), EP 0 571 868 (Wolff Walsrode) or EP 0 603 680 (Wolff Walsrode). The structures described in these documents may be incorporated into laminating films as the higher-melting layer or layers or have already been incorporated into known laminating films of this type. The production of TPU films using substantially incompatible polymers as flatting agents in TPU elastomers is also described, for example, in DE 41 26 499 (Elastogran).
Laminating films for industrial applications preferably have better hot-bonding properties on one side. Their bond properties are moreover usually specially tailored to the substrate onto which they are to be bonded. The hot melt adhesive layer of the laminating film is preferably selected from the same class of materials as the substrate. For industrial applications, the raw materials for the hot melt adhesive layer are thus preferably selected from the group of raw materials comprising the classes of thermoplastic polyamides, polyesters and polyurethanes. Conventionally, these hot melt formulations are not homopolymers, but instead copolymers. Decisive material characteristics such as melting point, softening point, crystallisation behaviour and modulus of elasticity may be adjusted by copolymerisation. Within the stated classes, both more elastic and more rigid formulations are known.
Prior art hot melt raw materials offered for sale, which are available for film production, are conventionally offered for sale as granulated or powdered products. Since they are often also used as raw materials for the formulation of crosslinking adhesives, they preferably have hydroxyl end groups. If the Kofler hot bench softening range of the lower-softening layer is distinctly above 100.degree. C., it is possible by means of purposeful cooling of the low-melting layer to produce the low-melting layer without an additional release layer. Such films made from polyamide are described, for example, in U.S. Pat. No. 3,762,986 (Allied Chemical Corporation). Corresponding films with a layer of copolyamide and a layer of TPU are described in EP 0 382 236 (BF Goodrich).
However, there is often a requirement to select the hot melt adhesive or heat sealing raw materials for the lower-softening layer such that they have a comparatively low softening range of below 100.degree. C., so that they do not damage the material to be laminated or provided with a skin during processing. Such a requirement is common for the provision of a skin on foamed plastics. Hot melt adhesives with Kofler hot bench softening ranges of below 100.degree. C. are thus conventional for this application. The melting points or melting range maxima of the lowest softening, commercially available adhesive raw materials are around 50.degree. C., measured to DIN 53 736. Due to the molecular weight and crystallite size distributions conventional in plastics, the melting or softening range of such a material extends over a wide temperature range of often up to 20.degree. C., which means that these materials may have an appreciable tendency to stick or block even at relatively high ambient temperatures and should thus often be classed as permanently tacky.
If laminating films are to be produced with a layer of very low-melting polymer resins, a release layer is often necessary for passage over rollers during production and/or in order to prevent blocking on the reel, because using such permanently tacky materials means that a web coated on one side with such an adhesive layer and wound into a roll, reel or winding cannot be unwound from the roll again because the front of the film sticks to the reverse. A permanent bond on the reel need not be formed to impair the processability of such a roll. In elastic materials such as TPU, even slight increases in the force necessary for unwinding may mean that the materials are so severely deformed under the tensile load that they can no longer reliably be processed.
In order to overcome the stated disadvantage, the free surfaces of the adhesive layer in known prior art elastic laminating films are provided with backing or release webs, which may readily be removed before the web is processed or bonded to a substrate. According to the prior art, the term backing or release layer is taken to mean a web which covers at least one of the sides of known prior art laminating films and is wound together with the laminating film, in order to prevent sticking or blocking of the individual turns of the roll on the reel. When the laminating film is unwound from the roll, the release layer remains on one side of the laminating film, but must be removed before the laminating film is put to its intended use.
If laminating films are to be produced which are provided with a heat sealing or hot melt layer which softens at below 100.degree. C., at least one release layer is always necessary according to the prior art in order reliably to prevent blocking on the reel. Such structures made from TPU are described, for example, in DE 1 964 059 Kalle). JA 61 086 262 (Toppan Printing), EP 0 002 465 (Bayer) or EP 0 024 740 (Plate Bonn) describe further processes for the production of such films. The last of these documents even describes the possibility of providing the laminating film with a release or protective film on both sides.
In the simplest case, polyethylene films may be used as release layers for laminating films having at least one layer from the group of the above-stated engineering thermoplastics, the polyamides, polyesters and TPU, which polyethylene films, even when unsiliconised, do not permanently adhere to polyurethane, copolyamide and copolyester hot melt pressure sensitive adhesive films and may thus be peeled off without further treatment. Prior art release layers may also consist of papers, films or other webs provided with silicone or other usual non-stick materials such as, for example, polytetrafluoroethylene.
The release layer materials must be produced separately, so increasing the price of the laminating films offered for sale. They are generally not wanted by the processor, who must separately unwind and often dispose of the release layer. They thus increase the cost of using laminating films and constitute an additional environmental impact.
Due to the dull, low-slip nature of the smooth surfaces of the low-softening layer of laminating films, various precautions must be taken during production and (further) processing in order to ensure rapid and unproblematic guidance of the film webs during production, use or conversion. One problem associated with the use of dull films is the formation of creases, which arise when tension is applied to the web and must be smoothed out before winding onto the reel or before contact with the substrate. Methods which may be considered for this purpose include inter alia the use of coated rollers and the greater than usual use of spreader rolls. In this connection, the processor in particular has considerable interest in a simplified and thus lower cost method of web guidance.
As an alternative to using release layer materials, it is also known to provide tacky or readily blocking materials with lubricants in such a manner that slip is achieved in machines and blocking on the reel is prevented. Using such materials in the lower-softening layer of the laminating film modifies the adhesive properties of this layer. The systems known for this purpose have the disadvantage that, due to surface occupation, they modify the adhesive or heat sealing properties or strength in such a manner that they do not achieve the initial performance of the unmodified material, even if processing parameters such as temperature, pressure and time are adjusted. Such lubricant, slip or non-stick systems are described, for example, by Brotz in Taschenbuch der Kunststoff-Additive, Gachter, Muller (eds.), 2nd edition, Hanser Verlag, Munich 1983.
It is also known to emboss fine structures on one side of the film, such that the contact area between the front and reverse of the laminating film is minimised on the roll. However according to the prior art, it is necessary to add a small proportion of wax to at least one layer in order reliably to prevent blocking. Here too, an unwanted reduction in adhesive properties occurs due to migration of the wax to the film surface.
The object of the present invention is accordingly to provide a laminating film for providing a skin on porous materials, which film may be stored without a release layer on the higher-melting or lower-melting side, without blocking on the reel and without losing its hot-bonding properties. The film should moreover be easier to pass through conventional machinery than known prior art laminating films. It should also be possible to process them under the conditions conventional for known laminating films. Such conditions include in particular processing on thermal laminating calenders, processing on flat bed thermal laminators and bonding activated laminating films to substrates in a press downstream from the activating station.